Electronic information-carrying signals often include various forms of error or distortion which must be corrected. For example, in the case of a modem, the information signal is transmitted via telephone lines from a transmitter to a receiver and the telephone lines introduce various forms of distortion into the signal.
Phase error is one common form of error existing in the received signal. To reduce the phase error, receivers typically include a phase lock loop which makes phase corrections which tend to reduce or eliminate phase error. Phase lock loops are shown by way of example in Motley et al U.S. Pat. Nos. 3,971,996 and 4,061,977.
Another form of signal distortion is phase jitter which may be considered as incidental frequency modulation. With phase jitter, the phase vector representing the information in the signal shifts. Noise may also be present in the signal.
The amount of signal disturbance resulting from noise and the amount of signal disturbance resulting from phase jitter are variable. Prior art phase lock loops do not provide optimum system performance under all phase jitter and noise conditions.